In order to transit smoothly from conventional telecommunication networks to next-generation IP networks, IETF (Internet Engineering Task Force) proposes a set of signaling transport (SIGTRAN) protocol model, for adapting various signaling messages in conventional telecommunication networks into IP networks, herein adapting specifically means replacing the previous information exchanges (including primitive, message and etc) between software modules put together with standard protocol so that two software modules can be separated completely without making modification. As shown in FIG. 1, signaling in conventional telecommunication network includes SS7 (Signaling System 7), DSS1 (Digital Subscriber Signaling System No.1), V5 Signaling, SCCP (Signaling Connection Control Part) and etc. All of these protocols take SCTP/IP as their lower-layer carrier protocol. For different signaling, the signaling messages can be transferred over IP networks by using different adaptation protocols according to practical requirements, and the carried signaling is setting on top of the adaptation protocol. The signaling data and command packet will be converted into corresponding adaptation protocol packet, and then be transferred to the destination in IP network via SCTP.
In this model, the SG (Signaling Gateway) on the border between conventional telecommunication network and IP network is responsible for conversion between signaling messages and IP packets. When there is any message to be transferred to SoftSwitch or MGC (Media Gateway Controller) in the SEP, the SEP (Signaling End Point) first sends the message to the SG through the signaling link between the SEP and the SG Upon receipt of the signaling message from its peer, the signaling carrier module in the SG reports it to the higher-layer via the internal interface, and meanwhile a fake higher-layer user module converts the signaling message into the corresponding different UA(User Adaptation) message according to the corresponding data and then sends UA message to the corresponding SoftSwitch or MGC over the established SCTP (Stream Control Transfer Protocol) connection. The UA module in the SoftSwitch or MGC recovers the received message and notifies it to the user applications of the higher layer through the internal interface, thus the signaling message is transferred from conventional telecommunication network to IP network. Contrarily, when the ISUP (ISDN User Part) or TUP (Telephone User Part) module in the Softswitch or MGC has any message to be transferred to the SEP in conventional telecommunication network, the message will be delivered to the UA module through the internal interface, and further transferred to the SG via SCTP. After obtaining the message received by the UA module, the fake high-layer user module in the SG sends it to the signaling carrier module in the SG through the same internal interface. Ultimately, the message arrives at the SEP through the signaling link between the SG and the SEP, and thus the transfer to conventional telecommunication network from IP network is achieved.
To adapt various signaling transfer into the new topologies in IP network, SIGTRAN protocols group introduces the notion of AS (Application Server) and ASP (Application Server Process) for all adaptation protocols. AS represents the collection of some specific signaling messages with advantage at shielding the difference in signaling ways, while ASP represents the process instance for handling the signaling message represented by the AS running on the SoftSwitch or MGC. After receiving a signaling message, the SG finds the AS to which the SG belongs according to the characteristics value carried in the message, then finds the available ASP according to the AS, and finally encapsulates the signaling message into IP packet and sends it to the SoftSwitch or MGC to which the ASP is pertained. Thus, signaling message distribution from signaling network to IP network is accomplished. An AS can be handled by one ASP, or by several ASPs simultaneously to achieve load share.
All adaptation protocol messages have ASPSM (Application Server Process State Maintenance) and ASPTM (Application Server Process Traffic Maintenance) messages with the same function, and the two messages are for implementing signaling link in IP networks. In addition, all adaptation protocols have SCTP as their carrier protocol. Therefore, all kinds of signaling links can utilize the same technique to realize redundancy.
There have been some discussions and considerations for link redundancy in current protocols, and the physical model is illustrated in FIG. 7. Every SG can connect several MGCs and every MGC can connect several SGs too. For those traffics that belong to the same AS, the SG can hand them to several MGCs for handling respectively according to the specific redundancy rule. The traffic messages of the same MGC can also be handed to different SGs for delivery according to the specific redundancy rule. Thus, it can be seen that the redundancy considered in the protocol is based on the whole network topology in such a basic way that two reliable connections are established respectively as the primary and backup links. In normal condition, the connection of the primary link is for data transmission and reception while the backup data is not for data transmission and reception. When a device has malfunction or doesn't work properly, all traffics will be handled by the backup link. Since different connections over IP networks are very likely to traverse across different networks, there can be great difference between the transfer rate of the primary link and that of the backup link. During switch, the late transmitted data may arrive earlier, which can lead to data disorder and loss. Under this situation, the protocol has not yet offered a good solution to avoid the data loss and disorder during switch.